Study Of Phase-shifiting Digital Holographic Microscopy Technology Based On Reflective Point Diffraction

As a simple on-axis configuration,point diffraction digital holographic microscopy has been an important test and analysis method in 3D microscopic imaging because of its whole-field,quantitative,non-contract,and label-free ability.Especially,it can achieve antidisturbance measurement.However,there are also some drawbacks for traditional point diffraction digital holographic microscopy,such as complex pinhole assemblies and difficult operations to introduce phase shifts.The digital holographic microscopy based on reflective point diffraction(RPD-DHM)which develops recent years,can effectively solve the problems by using a modified Michelson architecture to separate the object and reference beams.However,to simplify system complexity,take system measurement resolution and speed into consideration are still hot topics in the research field,and have scientific significance and practical values.The subject “Study of phase-shifiting digital holographic microscopy technology based on reflective point diffraction” is to develop new RPD-DHMs with simple configurations,less restrictive limitations,and high speed.Quantitative and stable measurements can be achieved by introducing high speed temporal phase-shifting,simultaneous phase-shifting and carrier phase-shifting technologies based on the analysis of the polarizing modulation characteristic of polarizing beamsplitter(PBS)and the phase-shifting characteristic of grating.Meanwhile,the measurement efficiency,and the utilization of field of view and spatial bandwidth of camera are taken into consideration.This study can provide new theories and methods for digital holographic microscopic imaging with high resolution,high efficiency and long-term stability.This study can obtain a wide application prospect in 3D microscopic imaging,such as micro-nano manufacturing,biomedicine,and so on.The main contents of this study are as follows:Firstly,some fundamental theories of digital holography based on reflective point diffraction are studied.To speed up the phase retrieval of off-axis RPD-DHM,a fast phase retrieval method using a specimen-free interferogram is proposed based on the analysis of off-axis PRD-DHM.A specime-free inteferogram is used to record carrier,carrier error and system aberration,and the retrieval speed is improved by the division operation to eliminate phase tilt and aberration without extra operations such as spectrum shifting,phase unwrapping and phase compensation.Then,a temporal phase-shifting point diffraction digital holography technology using a programmable reflective grating is researched to speed up the phase shift acquisition of temporal phase-shifting RPD-DHM.A digital micromirror device(DMD)is used to simulate the rapid phase shift of a reflective grating and achieve high precision and fast retrieval.Secondly,to simplify the system complexity of simultaneous phase-shifting RPD-DHM,a simultaneous phase-shifting point diffraction digital holographic microscopy using a PBS is proposed.Based on the analysis of polarization modulation of the object and reference beams with different polarizations,a 45o tilted PBS is used to separate the horizontal and vertical components of the 45° linearly polarized object beam and the circularly polarized reference beam,and obtain a phase shift of ?/2 between the two interferograms.Then,a modified two-step algorithm can be used to achieve on-axis phase-shifting measurement.With the analysis of carrier phase-shifiting geometry,slightly off-axis phase-shifting measurement can be achieved with the phase retriveal method using a specimen-free interferogram.The method avoids the limitation of intensities of the object and reference beams while the spatial bandwidth utilization of camera and acquisition speed can be ensured.Thirdly,to compromise the utilization of field of view of the camera and measurement speed,and avoid the experimental limitation of on-axis simultaneous phase-shifting technology,a two-shot phase-shifting point diffraction digital holographic microscopy using a reflective grating is proposed.Using the splitting and modulating characteristic of PBS,two phase-shifted interferograms can be captured in the first shot.In the second shot,another two phase-shifted interferograms can be acquired using the phase shift of ? introduced by the grating displacement.A four-step algorithm can be used to retrieve the phase.So,it provides an effective tool for digital holographic microscopic imaging with high speed,high resolution,and low complexity.Additionally,to further simplify the system complexity of simultaneous phase-shifting RPD-DHM,a simultaneous phase-shifting point diffraction digital holographic microscopy using a reflective grating is proposed.With the analysis of phase-shifting characteristic of different diffraction orders of the grating,the phase-shift of the 0 and ±1 diffraction orders are used.Three phase-shifted interfergrams can be captured in one shot and a normalized algorithm can be used to retrieve the phase.The system is greatly simplified without other beam-splitting elements,polarzing elements and phase-shifting elements.Finally,to speed up the retrieval of slightly off-axis carrier phase-shifting digital holography,a fast retrieval method using spectrum cropping is proposed to cut the redundancy in the spectrum domain with the analysis of carrier phase-shifting theory.Based on that,a retrieval method using spatial phase-shifting is proposed by phase shifting the two subtracted interferograms in the spatial domain to reduce the number of Fourier transform.Toimprove the utilization of the spectrum,a retrieval method using complex encoding is proposed by encoding the two subtracted interferograms into the real part and the imaginary part of a complex interferogram.The experiment results indicate that the three methods can all achieve the reconstruction speed more than 30 frames per second when the interferogram size is not greater than 768×768 pixels.So,the measurement speed is effectively improved.